One characterization of the efficiency of a rocket propelled vehicle, particularly one having a chemical propulsion system, is the mass ratio MR of the vehicle defined as the final mass of the vehicle (after propellants are consumed) over the initial mass of the vehicle (before propellants are consumed). The higher the MR, the more total vehicle and payload mass are carried by a unit of propellant, and the less propellant is used merely to carry later-consumed propellant. For a given MR, the proportion of the final vehicle mass available for payload may be increased by weight reduction techniques in the non-payload components, such as use of composite materials and the like.
MR itself can be improved by staging, which allows portions of the system no longer useful to be discarded so that later stages accelerate a smaller total mass. Staging adds significantly to the complexity of a propulsion system, however.
MR may also be improved by increasing the specific impulse (I.sub.SP) of a propulsion system, with I.sub.SP defined as the pounds of thrust divided by the mass flow rate in pounds per second of propellant exhaust flowing from the engine of the system, or as the seconds of pounds of thrust per pound of propellant materials. MR increases exponentially as a function of I.sub.SP. Simply put, the longer and larger the thrust provided from a given weight of propellant, the more propellant energy is available to accelerate the payload and other system components. The lsp of a system can be improved in various ways, including increasing the exhaust velocity of the system such as by increasing the reaction temperature and/or pressure, and increasing the completeness of the reaction of the propellant substance(s). All such improvements should be achieved with minimum added weight, however, or even with weight reduction, if possible.
Liquid fueled rocket propulsion systems generally require some means of delivering propellant to a thrust chamber under pressure. Stored pressurized gas may be used to drive propellant to the thrust chamber (known as a "blowdown system"), but the storage and handling systems for the gas take up valuable space and weight allotments. The required mass of the propellant tankage is proportional to the pressure applied to the propellant, which must exceed the chamber pressure of the engine. Since high chamber pressures are required for maximum I.sub.SP, such systems are penalized by the increased tankage mass. One or more pumps may be used to pressurize the propellant, but weight must be minimized and efficiency maximized. Such pumps require energy, in most advanced engines this is provided by turbines powered by the fuel and oxidizer, through combustion or as a byproduct of heating through regenerative cooling.
Rocket propulsion systems that are intended to operate continuously for significant lengths of time further require cooling of the combustion chamber and thrust nozzle. Such cooling should be performed as efficiently as possible with a minimum consumption of energy and/or propellant.